Shale gasses have become an economically important source of hydrocarbons in the early twenty-first century, as other reserves have run low and exploration companies have begun to look for new supplies of energy. In conventional oil and gas reserves the hydrocarbons are usually trapped within pore spaces in discrete pocket, usually of sandstone or porous limestones, which can be drilled into to release the hydrocarbons. In shales the gasses are found throughout larger, essentially impermeable, units, and need to be released proactively. This can be done through a process known as Hydraulic Fracturing, or Fracking, in which water, sand and chemicals are blasted into the rock at high pressure, causing it to shatter and release the gas. This has proved to be highly lucrative in some parts of the world, notably the US where it has fueled a small hydrocarbons boom, but has also caused great alarm among environmentalists, who linked the process to small Earthquakes and pollution of important aquifers, as well as raising concerns that making large amounts of new hydrocarbons available will lead to further rises in atmospheric CO₂, with consequences for the global climate.
As in other countries, there is growing interest in developing shale gasses as a source of fuel in the UK, though the extent of potentially exploitable reserves is largely a matter of speculation. One of the geological units that has been of particular interest is the Bowland-Hodder Carboniferous Shales, which underlie much of northern England. With this in mind the Department of Enregy and Climate Change
and British Geological Survey
have produced a report
into possible gas reserves within the Bowland Shales, published on 27 June 2013. This report does not make any judgement on the ethics of shale gas exploitation, or wether it would be possible to extract these gasses at all, but simply to assess the extent of the reserves in order to facilitate further discussion. In the absence of such published information governments have sometimes naively (or possibly dishonestly) underestimated the value of mineral reserves within their territories and granted concessions to companies at far less than their actual worth.
Location of the DECC/BGS study area in central Britain, together with prospective areas for shale gas, currently licensed acreage and selected urban areas. Other shale gas and shale oil plays may exist. DECC/BGS (2013).
Natural gasses are formed from the 'cooking' of biological material (principally plant matter) in deeply buried sediments. When biological material is sufficiently heated and pressurized, then volatile compounds are forced off as gas. In porous sediments this will typically escape from its point of origin, rising up until it becomes trapped at a high point in the porous rock, with impermeable sediments, such as clays, above. However when such material is trapped within clays or shales (finely laminated clays) it is unable to escape, and remains in situ. In order for gasses to develop in such sediments they need to be deeply buried, but they may subsequently be uplifted and found closer to the surface.
The Bowland Shales and underlying Hodder Mudstones were laid down in shallow marine basins in the early Carboniferous (347-318 million years ago), a time when Britain was close to the equator and sea-levels were rising and falling with the spread and shrinkage of ice caps. These deposits are not shale and/or mudstone throughout, rather there are layers of limestone within, formed as reefs which formed close to the shore spread back and forth with the rising and lowering level of the sea. These deposits have been deeply buried since the Carboniferous (which ended around 299 million years ago), and for the most part are still at some depth, though in places they have been uplifted and outcrop at the surface.
Shales from the Hodder Mudstone Formation outcropping on the flank of Ashnott High, Bowland Basin, Lancashire. Nicholas Riley in DECC/BGS (2013).
In order to assess the amount of gas trapped in these deposits, it was necessary to asses the amount of organic material within the sediment (estimated as typically 1-3%, but rising to 8% in places), as well as the thickness of the deposits. As these deposits are for the most part very deeply buried (the top of the unit is 4753 m bellow sea level at its deepest), they were only directly accessed by boreholes in a limited number of places, with much of the study being based upon geophysical data, principally from seismic studies and gravitational data.
The depth of the top of the Bowland-Hodder Unit below sea-level. Note that in places this is a negative figure; the deposits are above sea-level but still underground. DECC/BGS (2013).
Previous estimates of the thickness of the Bowland-Hodder Unit have ranged from 2500 m to 4000 m. The DECC/BGS study estimates it to be around 3575 m at its thickness, thinning down to zero in places.
The thickness of the Bowland Hodder Unit; it was not possible to measure map the unit in Derbyshire. DECC/BGS (2013).
However the volume of the unit cannot be correlated absolutely to the amount of gas it will contain. In order for organic material to have been converted to gas it will have to have been buried deeply enough for the process to have occurred, considered to be sediments that have been buried to a depth of 2900 m for the purposes of this study (even if they have subsequently been uplifted or exposed). This is referred to as the 'gas window', and can be mapped as a discrete horizon in itself.
The depth bellow modern ground level of the 'gas window' in the Bowland-Hodder Unit. Below this level the unit can be expected to contain shale gas, above it this is not likely to be the case. DECC/BGS (2013).
Thus the maps of the depth and thickness of the Bowland-Hodder unit do not give a reasonable estimate of the distribution of gas-bearing sediments in the study area. Furthermore sediments at depths of less than 5000 ft (1500 m) are not considered to be workable by hydraulic fractionation; the pressures in the rock are too low for the method to work.
Thickness and distribution of shales of the lower Bowland-Hodder unit that are within the gas window and at a depth greater than 5000 ft (1500 m). DECC/BGS (2013).
Thickness and distribution of shales of the upper Bowland-Hodder unit that are within the gas window and at a depth greater than 5000 ft (1500 m). DECC/BGS (2013).
The study produced three estimates of the total volume of gas in the rock (not the same as the total recoverable gas), referred to as the low, central and high estimates. The low estimate predicts a total volume of 4.6 trillion m³ of gas in the lower unit and 18.6 trillion m³ of gas in the upper unit, for a total of 23.3 trillion m³ of gas. The central estimate predicts a total volume of 7.5 trillion m³ of gas in the lower unit and 30.2 trillion m³ of gas in the upper unit, for a total of 37.6 trillion m³ of gas. The high estimate predicts a total volume of 12.7 trillion m³ of gas in the lower unit and 51.9 trillion m³ of gas in the upper unit, for a total of 64.6 trillion m³ of gas.